1. Field of the Invention
This invention relates to an apparatus for loading fuel rods into a fuel assembly for use in a nuclear reactor, such as a pressurized water reactor or the like.
2. Technical Background of the Invention
A conventional fuel assembly such as the one disclosed in U.S. Pat. No. 5,068,081, shown in FIG. 10, is known.
In this figure, the numerals 1 and 2 refer to top and bottom nozzles, respectively, which are disposed vertically and oppositely spaced apart, and have a plurality of rigidly fixed control-rod guide pipes 3 (hereinbelow referred to as guide pipes 3) between the top nozzle 1 and the bottom nozzle 2. In the mid section of the guide pipes 3 are a plurality of grids 4 disposed vertically and spaced apart from each other.
The grids 4 are constructed of a plurality of straps 7 made of thin metal strips having slits 8 formed in the longitudinal direction, as shown in FIG. 4. The slits 8 are interlocked to form a latticed structure as shown in FIG. 5, and each space bounded by the straps is known as a grid cell 5. Each grid cell 5 is provided with dimples 9 and springs 10 formed on the opposing walls of the grid cells 5. A fuel rod 6 inserted into a grid cell 5, shown in FIG. 6, is pressed against the dimple 9 by the spring 10, thereby holding the fuel rod 6 firmly in the grid cell 5.
There are different approaches to the steps involved in assembling such an assembly. For example, in the first method, as shown in FIG. 1, the grids 4 are disposed at a predetermined spacing, and the guide pipes 3 and an instrumentation tube is inserted into and fixed to designated grid cells 5 of each of the grids 4, respectively, followed by insertion of the fuel rods 6 into the other grids cells 5 of each of the grids 4 which are supported by the guide pipes 3. In the second method, the fuel rods 6 are inserted into and firmly held in the grid cells 5 first, followed by insertion of the control-rod guide pipes 3 and an instrumentation tube into the other grid cells 5 of the grids 4, followed by a step of rigidly fixing the guide pipes 3 to the grids. A method of inserting fuel rods 6 in grid cells 5 with the use of a pull-in loader is known, for example, in Japanese Patent Application, Laid open publication, H2-181,699, which involves the use of key means to deactivate the springs 10, followed by extending the pull-in rods of a pull-in device through the grids 5, gripping the tips of the fuel rods 6 housed in a fuel rod magazine with the gripping device attached to the pull-in rods, and loading the fuel assembly with fuel rods 6 by pulling the fuel rods 6 into the grid cells 5 of the grids 4.
There are problems in loading the fuel rods 6 with the use of pull-in rods after the guide pipes 3 (and an instrumentation tube) are fixed to the grids 4. One of the problems is the mechanical interference which occurs between the pull-in rods and the installed guide pipes 3 when trying to load the fuel rods 6 into the grids 4. The mechanical interference is due to the fact that certain grid cells 5 are already occupied by the cylindrical insert parts of the guide pipes 3 disposed at the entrance to the grids. In such a system, those pull-in rods corresponding with the grid cells 5 occupied by the guide pipes 3 must not be activated, and only those pull-in rods which correspond with the fuel rods 6 to be pulled into the grid cells 5 should be activated. The result is that the loading mechanism becomes complex because of the need for a special control device to select the correct pull-in rods. Furthermore, because the arrangement of the guide pipes in the grids are different for each tier of grid cells, the operation of the pull-in rods must be adjusted for each tier of grid cells, leading to a lengthy and cumbersome fuel rod installation operation.